Experimental And Clinical Studies Of Interbody Fusion In Lumbar Spine

Spinal instability is one of the important factors for the low back pain and fusion of lumbar spine is a well-accepted surgical procedure in the management of reconstruction of spinal stability. Interbody lumbar fusion has gained wide popularity in treatment of lumbar instability for its advantages of high fusion rate and immediately restoring of the spinal stability. Recent developments of interbody fixation devices known as cages have renewed interest in lumbar interbody fusion. However, there were some intro-and post-operative complications occurred with the increasing use of interbody fusion cages. In this paper, a retrospective study of low lumbar instability treated by interbody fusion was conducted and the long-term follow-up of functional outcomes, radiographic changes and the complications were analyzed. To avoid these complications, a prospective research of using newly designed titanium-coating fusion cages in the treatment of lumbar instability was performed. Subsidence of the cages into the vertebral body is a well-known complication after interbody fusion, especially in the old patients with low bone mineral density, so biomechanical test was hold to study the effect of endplate preparation and bone mineral density on the compressive strength of the graft–endplate interface and initial stability of fusion spinal unit. Utilizing vivo animal models, the ability of hydroxyapatite-silk fibroin nanocomposites to act as bone graft substitutes for interbody lumbar fusion has been studied. Part Ⅰ. Long-term effects of lumbar instability treated with lumbar interbody fusion. Objective To investigate the long-term effects and complications of lumbar interbody fusion for lumbar instability. Methods From 1996 to 2003, 66 cases of 75 segments with lumbar instability were followed up after underwent interbody lumbar fusion using cages. The contents followed up included: (1) Radiological studies: The average preoperative and postoperative disc space height, foraminal height, lumbar lordosis, segmental lordosis and the amount of sagittal rotation observed between the extremes of movement (on flexion–extension radiographs) were measured and compared; (2) Clinical outcome evaluation: pre-and postoperative JOA score and Oswestry Disability Questionnaire were used to determine the long-term effect of the operation. Intro-and post –operative complications were also accessed. Results All cases were followed and the follow-up period ranged from 18 to 70 months with an average of 26 months. The fusion rate is 74.3%. The recovery rate was 70.6% (ALIF group) and 72.9% (PLIF group) and the ODI decreased significantly. There were significant retrieve of disc space height and foraminal height, and the regress of the amount of sagittal rotation was attained, but the satisfied postoperative sagittal alignment was not achieved. In some cases, losses of retrieved disc space height accompanied by reoccurrences of the neurological symptoms were observed. The migrations of the fusion cages backwards into the spinal canal and subsidence into the vertebral body were occurred in 3 (4.5%) and 11 (16.6%) cases respectively. Conclusions (1) Lumbar interbody fusion is a reasonable option in treating the lumbar instability; (2) Normal sagittal alignment was not achieved which will accelerate degeneration in adjacent segment; (3) Postoperative subsidence of the fusion cages which lead to losses of the disc space height at the fusion level will alter the mechanical results and possibly adversely affect the clinical results, and it is associated with design of the cages, the preparation of endplate and the bone mineral density of the patients. Part Ⅱ. Clinical applications of titanium coating fusion cage in treatment of lumbar instability. Objective To study the clinical outcomes of using titanium-coating cage (Prospace) in lumbar segmental instability. Method From January 2003 to October 2004, a prospective clinical research was designed that 20 cases with 21 segments diagnosed as lumbar instability were treated by posterior lumbar interbody fusion (PLIF) using Prospace cage. 4 males and 16 females with average age of 51.6 years were included. The diagnoses were disc herniation in 5, lumbar spinal stenosis in 6,degenerative spondylolisthesis in 4 and isthmic spondylolisthesis in 5. In all cases, double cages were inserted with the posterolateral bone graft and pedicle screw systems were supplemented. The average preoperative and postoperative disc space height, foraminal height, lumbar lordosis, segmental lordosis, lumbar tilt, sacral tilt, Index of Lumbar Curvature and the amount of sagittal rotation observed between the extremes of movement (on flexion–extension radiographs) were measured and compared. The JOA score and Oswestry Disability Questionnaire were used to evaluate the clinical outcome. Result All cases were followed and the follow-up period ranged from 6 to 20 months with an average of 12 months. The fusion rate is 95%. The recovery rate was 91.6%and the ODI decreased significantly. There were significant retrieve of disc space height and foraminal height, and the restoring of the amount of sagittal rotation observed between the extremes of movement was achieved. Mean Lumbar lordosis and segmental lordosis of the fused segments showed significant changes postoperatively (P<0.05). Segmental lordosis increased from 5.1°to 10.1°at L4,5, and from 9.2°to 16.6°at L5S1, lumbar lordosis increased from 31.2°to 41.1°, and Index of Lumbar Curvature increased from 25.1% to 30.5%. Analysis of changes in lumbar tilt and sacral tilt did not show significant differences. Conclusion(1)The restoration of the interbody space height and achievement of optimal sagittal alignment of the lumbar spine are relevant factors influencing the clinical outcomes. (2) Compared with threaded fusion cages, the anatomic titanium-coating fusion cage has its advantages in restoring sagittal alignment and keeping bony endplate intact which can prevent the postoperatively subsidence in instrumented PLIF procedures, at the same time, it can accelerates interbody fusion due to an osteointegration of the vertebral endplates by titanium coating without additional bone grafts. Part Ⅲ. Effect of bone mineral density and endplate preparation on the initial stability and compressive strength of the fusion spinal unit Objective To investigate Effect of bone mineral density (BMD) and endplate preparation on the initial stability and compressive strength of the fusion spinal unit. Method Using the goat underwent ovariectomy as animal models of low bone mineraldensity. Fusion unit (L4 ,5) and single vertebral body samples were prepared respectively and bone mineral density of each sample was measured using a dual energy radiograph absorptiometry unit. Fusion unit specimens were divided into four groups according to BMD and different endplate conditions (intact and completely removed) and single vertebral body specimens were divided into six groups according to BMD and different endplate conditions (intact, completely removed and one central 6×6mm2 hole). The biomechanical evaluations include compressive strength, rigidity, flexibility; extension, axial rotation and destructive compression tests were performed in this study. Result (1)The compressive strength, rigidity and initial stability of fusion spinal unit were found to have a significant association with bone mineral density and preparations of the endplate while group with normal BMD and intact bony endplate has the best initial stability. (2) In single vertebral body specimens, load to failure of the specimens with an intact endplate and normal BMD were significantly greater than that of the specimens with removed endplate or low BMD. One central hole (6×6mm2 ) doesn't significantly decreased the compressive strength of the endplate. Conclusion(1)BMD and endplate condition significantly affect the initial stability of the fusion spinal unit, (2) BMD and endplate (intact vs. complete removal) were found to significantly affect the mechanical strength of the graft-endplate interface and making a central hole in the endplate to facilitate the vascular ingrowths to the graft is preferable. (3)To reduce the incidence of postoperative complications such as subsidence and migration of the cages when performing interbody fusion, preoperative consideration and postoperative interference of BMD would be important for patient selection and the choice of surgical technique. Preserving the intact bony endplate as much as possible is necessary, particularly in patients with poor bone quality. Part Ⅳ. The study of using Hydroxyapatite-Silk fibroin nanocomposites as bone graft substitutes for lumbar interbody fusion Objective To evaluate the ability of osteoconduction of Hydroxyapatite-Silk fibroin nanocomposites(HAp-SF) and its potential efficacy as graft substitutes for autogenous bone in lumbar interbody fusion. Method Five goats underwent single-level(L3,4) anterior lumbar interbody fusion (AILF) and the HAp-SF nanocomposite block was implanted as graft substitutes for autogenous bone. The animals were sacrificed after 5 months and spinal fusion was evaluated by manual palpation (status was fused or not fused), and radiological analyses include plain X-ray, computed tomography and MRI was done to visualize the amount of bone formation. Histological examination was performed to observe the new bone formation. Result Fusion was ascertained by manual palpation in all animals, and the radiological analyses show the new bone formation and solid fusion in the interbody space. Histological examination suggested that newly born bone matrix and blood vessels were observed in the fusion segment. Conclusion As graft substitute for autogenous bone in lumbar interbody fusion, Hydroxyapatite-Silk fibroin nanocomposites had showed its satisfied osteoconductive properties and it is promising in future clinical application.